The present invention is related to a frequency synthesizer, and more particularly, to a direct digital synthesizer (DDS).
Direct digital synthesis (DDS) relates a technique of frequency synthesis, which generates different waveform output according to different frequencies and clocks for providing the signal source in the continued circuit. The samples of the generated signal are prepared in digital mode and these samples are converted into analog form by means of a digital-analog converter (DAC). Direct digital synthesis is a well circuit technique widely used in frequency synthesizer for wireless application, which covers wide frequency ranges, very fast frequency hopped, or arbitrary waveform generation.
Please refer to FIG. 1. FIG. 1 is a block diagram illustrating the conventional direct digital synthesizer according to the prior art. The conventional direct digital synthesizer includes a phase accumulator 10, a wave lookup ROM-table 11, a linear digital-analog converter (DAC) 12, and a low pass filter 13. The wave lookup ROM-table 11 is used for the storage of different waveform parameter.
Conventionally, the phase accumulator 10 combines with the wave lookup ROM-table 11 to generate a digital waveform according to the frequency and clock input. The digital waveform is delivered to the linear digital-analog converter (DAC) 12 to form an analog signal. Then, the linear DAC 12 output analog signal is passed through the low pass filter 13 to get a desired waveform output.
However, the conventional direct digital synthesizer usually uses the binary weighted or the linear weighted DAC, which needs to combine with the wave lookup ROM-table to generate the digital waveform. Moreover, the chip size is increased and the circuit design is more complex in the conventional art.
In order to overcome the drawbacks in the prior art, a direct digital synthesizer for generating a waveform output is provided, which is able to eliminate the conventional wave lookup ROM-table and simplify the circuit design.
The main purpose of the present invention is to provide a direct digital synthesizer, which is able to eliminate the conventional wave lookup ROM-table and simplify the circuit design for direct digital synthesis.
It is one object of the present invention to provide a direct digital synthesizer, which is able to generate all kinds of periodic waveforms with different phases.
It is another object of the present invention to provide a direct digital synthesizer with ROM-less structure, which is easier to be implemented and has a reduced chip size.
It is another object of the present invention to provide a direct digital synthesizer, which is suitably used in the waveform generator and the digital modulator of wireless product because of the low cost.
According to one aspect of the present invention, a direct digital synthesizer (DDS) for generating a waveform output according to a frequency input and a clock signal, comprises: a code generator for generating a digital code in response to the frequency input and the clock signal, a digital-analog converter (DAC) electrically connected with the code generator and comprising plural difference weighted coefficients for generating an analog waveform through an operation of the plural difference weighted coefficients and the digital code, and a filter electrically connected with the difference weighted digital-analog converter (DAC) for filtering the analog waveform to generate the waveform output.
In accordance with the present invention, the code generator is a thermometer code generator.
Preferably, the digital code is a thermometer code.
Preferably, the digital-analog converter (DAC) is a non-linear difference weighted digital-analog converter.
Preferably, the digital-analog converter (DAC) is a current steer type DAC.
Preferably, the digital-analog converter (DAC) is a resistor type DAC.
Preferably, the filter is a low pass filter.
Preferably, the low pass filter is an anti-alias low pass filter.
Preferably, the waveform output is a periodic waveform.
Preferably, the waveform output is a sinusoidal waveform.
Preferably, the waveform output is an arbitrary waveform.
According to another aspect of the present invention, A method for a direct digital synthesizer (DDS) to generate a waveform output, comprises steps of: (a) defining plural difference weighted coefficients, (b) providing a frequency input and a clock signal, (c) generating a digital code in response to the frequency input and the clock signal, (d) generating an analog waveform in response to the digital code and the plural difference weighted coefficients, and (e) filtering the analog waveform for obtaining a desired waveform output.
The foregoing and other features and advantages of the present invention will be more clearly understood through the following descriptions with reference to the drawings, wherein: